Luis is 3 times as old as Omar and is also 10 years older than Omar. How old is Omar?
We can use the given information to write down two equations that describe the ages of Luis and Omar. Let Luis's current age be $l$ and Omar's current age be $o$ $l = 3o$ $l = o + 10$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $o$ , and both of our equations have $l$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $3o$ $-$ $ (o + 10)$ which combines the information about $o$ from both of our original equations. Solving for $o$ , we get: $2 o = 10$ $o = 5$.